Fuel burner control apparatus



June 5, 1956 M. R. SMITH FUEL BURNER CONTROL APPARATUS Filed Jan. 25,1952 ATTORNEY z: fi H92 8529 .m A I m w m 2N WM M 02 2 MS n WR. 96 N wd5 0 E w 58z8 99: m wexnjzw 35 M o w t m 8 wwm Y om mw B fivow 5 mom o-H n 8 m: I4 M3 r 1w A 9 wv o v w A 8 WM. AM

roar nuance coisraon APPARATUS Myron R. Smith, Minneapolis, MlIilL,assignor to Minneapolis-Honeywell Regulator Company, Minneapolis, Minn,a corporation of Delaware Application January 25, 1952, Serial No.263,270

6 Claims. (Cl. 25ll-83.3)

The present invention is concerned with an improved fuel burner controlapparatus for detecting a burner flame by sensing a particularcharacteristic of the flame.

It is well known that a burner flame is fluctuating in character, thatis, the electromagnetic wave energy emitted by the flame fluctuates inintensity and the conductivity which the flame presents to the flow ofelectric current fluctuates. The use of a photoelectric cell to detectthis characteristic fluctuation in the intensity of wave energy emittedby a flame is shown in the Harry S. l ones patent, 2,304,641, and theuse of a flame rod to sense the char acteristic fluctuation inconductivity of a flame is shown in the Walter P. Wills Patent2,352,143.

The detection of a flame by utilizing this characteristic fluctation inwave energy radiated by the flame is a decided advantage since flamedetectors utilizing this principle will not detect the radiant energygiven ofl by the flreboXes wherein the burners are located when thesefireboxes are heated to a luminous condition. This is true since theluminous character of the walls of the firebox are of a constantintensity and do not fluctuate as does the burner flame.

An additional improvement was made to this type of detector, whichdetected the fluctuating characteristic of the flame, when the flamedetector was used with a lead sulphide photo-conductive cell. type ofcell it was found that both oil and gas flames could be detected sincethe lead sulphide photo-conductive cell is sensitive to the infraredregion and both the gas and oil flame emit radiations which arepredominantly in the infrared region. The use of a conventional flameelectrode allows the flame detector to be used only to detect a gasflame since an oil flame tends to cause the flame electrode to becovered with an excessive amount of carbon and the flame electrode istherefore unable to withstand the excessive temperatures of an oilflame.

When the conventional photo-emissive cell, such as the cesium oxidesilver cell, is used to detect flame it has been found that only the oilflame can be detected since this type of cell is not sensitive to theradiations from a gas flame while it is sensitive to the shorterelectromagnetic waves radiated from the oil flame.

It has also been found that controls such as that shown in the abovementioned Jones and Wills patent operate more successfully if afiltering means is used to reject all control signals received from theflame sensing means which are not in the characteristic frequency rangeof fluctuation of the flame. It will be immediately recognized that theconductors running from the flame sensing means to the control will pickup stray signals, for example, a 60 cycle per second signal when thecontrol is used with a 60 cycle alternating current source of power. Theshielding of these conductors eliminates stray signals to a certainextent, however a filter means was used to insure that the control wouldbe sensitive to only control signals within the characteristic frequencyrange indicative of flame. It has been found that this characteristicfrequency of fluctuation in a flame is pre- When utilizing this ratesflatent ft "W 2 dominately in the range of from 10 to 30 cycles persecond It therefore became necessary to develop a burner controlapparatus which would be sensitive only to this low frequency range andyet had a simple filter means which would not burden the burner controlapparatus with an excessive number of components so as to render thecontrol apparatus acceptable in a relatively competitive market.

It is therefore an object of the present invention to provide animproved burner control apparatus having a flame sensing means to detectthe fluctuating characteristic of a flame and having a simple filternetwork rendering the flame detector frequency selective to only thefrequencies of fluctuation of the flame.

it is a further object of the present invention to pro vide anelectronic flame detector having a photoelectric cell to detect thepresence of flame and having associated therewith a filter networkconnected in a frequency selective degenerative feedback circuit torender the electronic flame detector sensitive to only that range offrequencies including the characteristic fluctuation in the flame to bedetected.

The copending application of James W. Smith et al., Serial Number268,182, filed January 25, 1952, shows a combustion safeguard apparatusutilizing a frequency selective amplifier in a novel adaptor unit whichis used with a conventional type electronic flame detector to provide anapparatus which responds only to the flickering characteristics of theflame. This copending application also discloses a novel structurewhereby a flame rod and a photocell are selectively connected to theconventional type electronic flame detector, the flame rod beingdirectly connected and the photocell being connected through the adaptorunit.

The single figure is a circuit diagram of the improved burner controlapparatus used in conjunction with a gas main burner and gas pilotburner.

The main gas burner it} is shown with a valve 11 controlling the flow offuel thereto. A pilot burner 12 is located in an igniting relationshipto the main burner 10 and has a valve 13 controlling the flow of fuelthereto. Associated with the pilot burner 12 is an ignition electrode 14connected to an ignition transformer 15 with the circuit for theignition transformer 15 being completed by a connection 216 to the pilotburner 12.

A lead sulphide photoconductive cell 16 is normally positioned to viewboth the flame emitted from the pilot burner 12 and the flame emittedfrom the main burner id. The photo-conductive cell 16 is connected bymeans of conductors l7 and 13, which conductors are shielded by a shield20, to the input terminals 21 and 22 of a control apparatus shown withinthe broken line 23.

The burner control apparatus 23 is under the control of a conditionresponsive means such as a thermostat 24 which is located in the spaceto be heated by the main burner it The thermostat 24 is arranged so thaton a call for heat, or a need for operation of the burner 1:9, a circuitis completed from terminal 25 to terminal 26 of the control apparatus23.

The control apparatus 23 includes an input stage including a firstelectron discharge device 27, an intermediate stage including a secondelectron discharge device 23 and an output stage including a thirdgaseous electron discharge device 29. The discharge device 27 comprisesan anode 30, a control electrode 31 and a cathode 32. T he dischargedevice 2-3 comprises an anode 33, a control electrode 34 and a cathode35. The gaseous discharge device 29, commonly known as a thyratron,includes an anode lli, a suppressor electrode 41, a control electrode 42and a cathode 43.

The discharge device 29 has as its power source a secondary 4 of atransformer 4-5 and is arranged to energize a relay winding 46 of arelay 47. Switch blades 48, 49, and 53 of the relay 47 are biased bymeans not shown to the position shown in the single figure when therelay 47 is not energized. Upon energization of the relay 47 the switchblades 43 and 59 move into engagement with stationary contacts 51 and 52respectively. The switch blade 49 on the relay 47 becoming energizeddisengages contact 53 and moves into engagement with contact 54.

The discharge devices 27 and 23 are energized by means of a directcurrent power source 66 having a selenium rectifier 61 connected to asecondary 62 of the transformer 45 to produce a direct current voltagesource wherein terminal 63 is rendered positive with respect to groundterminal 64.

Connected in circuit with the before mentioned input terminals and 26,which are arranged to be connected by the thermostat 24, is a relay 65having a winding 66. Switch blades s7 and 68 of the relay 65 are biasedby means not shown to assume the position shown in the single figurewhen the relay winding 66 is not energized. Upon the energization of therelay winding 66 the switch blade 655 moves into engagement withstationary contact 69 and the switch blade 67 moves out of engagementwith a stationary contact 79 and moves into engagement with stationarycontact 71.

Also in circuit with the relay winding 66 is a safety cut-out device 75.The safety cut-out device '75 comprises a bimetal actuator 76 having abimetal heater 77 associated therewith and having normally closedcontacts 78 and '79. Upon the heating of the bimetal warps to the rightand allows the contact 79 to disengage from the contact 73. Upon coolingof the bimetal 76 a reset actuator 88 can be depressed to again resetthe safety cut-out device and when the reset actuator is released thedevice will again assume the position shown in the single figure.

The winding 66 of the relay 65 is connected in circuit with the safetycut-out device 75 and the bi-metal heater 77 to be energized by asecondary winding 81 of the transformer 45. A center tap 82 is providedon this secondary winding 81. A primary winding 33 for the transformeris connected to terminals 84 and 85 which are adapted to be connected topower line conductors 86 and 87.

Referring particularly to the operation of the three electronic stagesof the control apparatus 23, the proper signal applied to the inputterminals 21 and 22 is amplified by the electronic discharge devices 27and 28 and their associated circuits and is applied to the controlelectrode 42 of the thyratron 29 to cause energization of the relay 47in the manner to be described.

As before mentioned, the lead sulphide photoconductive cell 16 issensitive to the infrared wave energy being radiated by the flame at thepilot burner 12 and the main burner 10. This infrared wave energyfluctuates in intensity in a characteristic frequency range of from 10to 30 cycles per second. This causes the conductivity of the cell 16 tovary in a characteristic frequency range of from 10 to 30 cycles persecond. A direct current source of voltage is supplied to the cell 16from the direct current source of power 69 through two graded RC filterscomprising a resistor 90, a capacitor 31, a resistor 92, and a capacitor93. This energizing circuit for the photoconductive cell 16 can betraced from the terminal 63 of the power supply 60 through conductor 94,resistor 9t), conducttor 95, resistor 92, conductor 96, load resistor97, input terminal 21, conductor 17 photoconductive cell 16, conductor13, input terminal 22, and ground connection 95 to the negative terminalof the power supply 6i) at ground connection 64. A fluctuating directcurrent fiows in this circuit when the photoconductive cell 16 isexposed to a flame and a functuating signal. voltage is therebydeveloped across the load resistor 97..

This fluctuating control signal is applied to the con-- trol electrode31 of the discharge device 27 through a blocking capacitor 99 and aresistor ltlil. This first ampli-- fication or input stage is aconventional stage making use of cathode biasing obtained by cathoderesistor 101 and capacitor 162. The fluctuating control signal is.amplified by the electronic discharge device 27 and is developed acrossa plate load resistor 1% which con meets the direct current source ofpower 60 to the anode: 30.

The amplified control signal developed across the plateload resistor 103is applied to the control electrode 34 of the discharge device 2%through the input circuit of the intermediate stage including a couplingcapacitor 1M and a resistor 105. The second amplifying or intermediatestage is also a conventional stage making use of cathode biasing bymeans of cathode resistor R06 and capacitor T 7. The control signal isagain amplified by the discharge 2'5 and is developed across a plateloadresistor 1'89 in. its output circuit which connects the: direct currentsource of power so to the anode 33.

The second amplifying stage of the control apparatus 23 as so fardescribed would amplify all signals applied to the input terminals 21and 22 and would cause the gaseous discharge device 29 to fireregardless of the frequency of the input signal. As before mentioned,the only signal which is indicative of the pressure of flame at thepilot burner 12 of the main burner it} lies within the frequency rangeof from it) to 30 cycles per second. It is therefore desirable to renderthe control apparatus 23 selective to frequencies lying only within thispredetermined range. Two things to be considered while selecting theproper circuit to render this amplifier as so far described frequencyselective are, first, that the amplifier must be selective to arelatively low frequency and second, that the means used to render theamplifier frequency selective must be relatively simple so as not tounduly burden the control apparatus 23 with the large number ofcomponents.

In the present invention it has been found desirable to connect a bridgeT null network from the anode 33 of the discharge device 28 to thecontrol electrode 34- of this discharge device. This bridge T nullnetwork is shown generally at 11% and has a first capacitive leg illconnected to the anode 33 of the disctarge device 28. A secondcapacitive leg R12 is connected through the coupling capacitor 104 tothe control electrode 34 of the discharge device 23. A third resistiveleg T13 is connected to ground or reference potential as the cathode 35of the discharge device 23 is connected to the reference potentialthrough the resistor m6. A bridging resistor 114 is provided whichshunts the first and second capacitive legs 111 and 112 respectively.

In the present invention it has been found desirable to select thefollowing values for the components of the bridge T null network.

Resistor 113 .2 megohm. Resistor 114 10 megohms. Capacitor ill .005 mcrofarad Capacitor 112 .095 microfarad.

The operation of this bridge T null network can be readily understoodwhen it is recognized that this network is connected to givedegenerative feed-back from the an ode 33 to the control electrode 34 ofthe discharge device 28. The nature of this feed-back is such that forall frequencies except that within a predetermined range, 10 to 30cycles per second, this second amplifier stage lS degenerative and thecontrol signal for these frequencies is not developed across the plateload resistor 169 of the discharge device 28. For control signals lyingwithin the range of from 10 to 30 cycles per second which are applied tothe input terminals ill and 22 the bridge T null network is not adegenerative feedback circuit and b a signal voltage of from 1 tofiocycles pertsecond isdeveloped across the plate load resistor 109.

The control signal developed across the plate load resistor N9 isapplied to the control electrode 4210f the dis charge device 29 throughblocking capacitor 120 and capacitor 121. A biasing circuit issupplied'for the discharge device as and can be traced from the controlelectrode 42 through a resistor 122, conductor 123, conductor 124,potentiometer R25, potentiometer tap 126,,ground connection 64, andground connection 127 to the cathode 43 of the discharge device 29. Thenature of the voltage present across the potentiometer 125 is such thata negative bias is supplied to the discharge device 29 and the positionof the tap 126 determines the sensitivity of the discharge device 29,that is, the greater the negative bias supplied to the device 2%, thegreater the magnitude of signal developed across the plate load resistor1199 necessary to fire the discharge device 29.

When a signal of the proper magnitude is developed across the plate loadresistor 109 thegaseous discharge device 29 will fire to energize therelay 47. Since alternating current power is supplied to the anodecathode circuit of the discharge device 29, pulsating current will flowthrough the winding 47 and a filtering capacitor 128 is provided acrossthe winding 46 to filter this pulsating current.

A set-back control is also provided for the discharge device 29. Thefunction of this set-back control is to reduce the direct current biasvoltage. on the discharge device 29 when the relay 47 becomes energizedand thereby insure that the relay 57 will be positively energized andthat the associated switch contacts will be positively moved to theiractuated positions. The set-back control comprises a potentiometer 1%having a movable tap 131. Assuming that relay 47 is energized, a biasingcircuit for discharge device 29 can be traced from the control electrode42 through resistor 122, conductor 123, conductor 124, conductor 132,movable switch blade 48 and contact 51 of relay 47, conductor 133,movable tap 151 of potentiometer 130, movable tap E26 of potentiometer125, ground connection 64, and ground connection 127 to the cathode 43of the discharge device 29. The potentiometer 13b is effectively inparallel with the potentiometer 125 and the effect of this parallelconnection is to reduce the negative bias voltage applied to the controlelectrode 42 of the discharge device 29 thereby increasing thesensitivity of the discharge device 29 once the relay 47 has beenenergized.

Operation it will first be assumed that the thermostat 24 is not tracedfrom the upper terminal of the secondary 81 through conductor 14%,terminal 25, thermostat 24, terminal 26, relay winding es, safetycut-out contacts '78 and 79, conductor 141, conductor 142, stationarycontact 53 and movable switch blade 49 of relay 47, conductor 143,safety cut-out heater 77, and conductor 144 to the tap 82 or" thetransformer secondary 81. It will be noted from this last traced circuitthat it is necessary for the relay 47 to be deenergized so that themovable switch blade 49 will engage stationary contact 53 in order forthe relay 65 to be energized. This is a component checking feature inthe improved burner control apparatus and insures that upon the relay 47being energized to falsely indicate flame it is impossible for the relay65 to be energized and, as will be explained later, unless the relay 65is energized the valves 11, 13 and the ignition transformer cannot benergized.

Energization of the relay 65 causes the movable switch blades 57 and asto move to their operative position. The movement of the switch blade 67to engage the contact 71 establishes a holding circuit for the relay 65which is independent lofthe "safety 'cut-out heater 77. This circuit canbe traced from the upper terminal of the secondary 81 through conductor140, terminal '25, thermostat 24, terminal 26, relay'winding 65,contacts 78 and 79 of the cut-out device '75, conductor 141, conductor15o, contact 71 and switch blade 6'7 of relay 65, and conductor'lSltothe lower terminal .of the secondary 81. However the heater 77 of thesafety cut-out device remains energized through a circuit which can betraced from a tap 82 of the secondary 81 through conductor 144, heater77, conductor switch blade 49 and contact 53 of relay 47, conductor 142,conductor 150, stationary contact 71 and switch blade 67 of .relay'65,and conductor 151 to the lower terminal of the secondary 81. In order todeenergize the heater 77 of the cut-out device 75 it is necessary forthe relay 47 to be energized thereby braking the above traced circuitwhen a switch blade 49 disengages from the stationary contact 53.

Energization of the relay 65 also causes the movable switch blade 63 toengage the stationary contact 69. This causes the valve 13 and theignition transformer 15 to'be energized by means of wiring which isexternal to the control apparatus 23. The energizing circuits for thevalve 13 and the ignition transformer 15 can be traced from the powerline conductor 36 through conductor 155, terminal 156, stationarycontact 69 and movable switch blade es of relay 65, terminal 157,conductor 158, conductors 159 and 16b to the valve 13 and the ignitiontransformer l5 respectively and conductors 161 and 162 from the valve 13and the ignition transformer 15 respectively to the power line 87.

Gas now passes to the pilot burner 12 and is ignited by means of theignition transformer 15 and the ignition electrode 14. The lead sulphidephotoconductive cell 16 senses the fluctuations in infrared radiationsemitted by the flame at the pilot burner 12 and supplies a signal in thecharacteristic frequency range of from It) to 30 cycles per second tothe input terminals 21 and 22 of the control device 23. In the matterabove described the relay 47 of the control apparatus 23 is energized asan indication of the presence of flame at the pilot burner 12. Themovable switch blades 48, 49, and 54) are therefore actuated to theirenergized positions.

Movable switch blade 48 engaging contact 51 completes the set-backcircuit for the discharge device 29 and increases the sensitivity of thedischarge device in the manner above described. The switch blade 49disengages from the contact 53 to deenergize the heater 77 of thecut-out device 75 in the manner above described. The switch blade 49then engages the stationary contact 54, the purpose of this arrangementis to provide a further safety feature in the improved burner controlapparatus which will be described later.

Engagement of the switch blade 50 with the stationary contact 52 iseffective to energize the valve 11 and thereby cause gas to flow to themain burner it This energizing circuit can be traced from the power lineconductor 87 through conductor 17b, conductor 2il2, terminal 2%,conductor 2&11, contact 52 and switch blade 56 of relay 47, conductor2&3, terminal 2434, conductor 172, valve 11, conductor 171, conductor16%, conductor 158, terminal 157, switch blade 68 and contact 69 ofrelay 65, terminal 156, and conductor 155 to power line conductor 86.

A flame will now be established at the main burner 10 and heat will besupplied to the space wherein the thermostat 24 is located.

Operation 0n flame failure Should the flame at the pilot burner 12 andmain burner 10 now accidentally become extinguished, the photoconductivecell 16 no longer senses the characteristic fluctuations of the flameand although electrical disturbances may be picked up in the shieldedconductors 17 and 18 and applied to the input terminals 21 and 22, thebefore mentioned bridge T null network connected to be degenerative toall frequencies other than the characteristic fluctuations of the flamewill prevent these dis- I turbances from firing the discharge device 29.A voltage relay 4'7 is deenergized.

The switch blade 48 disengages from the contact 51 and thereby decreasesthe sensitivity of the gaseous discharge device 29. The movable switchblade 49 engages the contact 53 to again energize the safety cut-outheater 77 in the manner above described. The movable switch blade 50disengages from the contact 52 to deenergize the valve 11 and therebyturn off the flow of gas to the main burner it). However, the energizingcircuit for the valve 13 and the ignition transformer 15 is uneflcctedsince relay 65 remains energized to retain the movable switch blade 68engaged with the contact 69 of relay 65, which switch blade and contactcontrol the energization of the valve 13 and ignition transformer 15.

A predetermined time after the deenergization of relay 47 the safetycut-out heater 77 heats the bimetal 76 to a point Where it warps to theright and allows the contact 79 of the cut-out device to disengage fromthe cou tact 7?. This deenergizes the relay 65 to cause the switch blade6% to disengage from the contact 69 thereby deenergizing the valve 13and the ignition transformer 15.

The opening of the cut-out device contacts 78 and 79 also deenergizesthe heater 77 of the cut-out device and a predetermined time thereafter,the bimetal 76 returns to its cool condition. An operator may thenactuate the reset button 80 to again reset the contacts 7% and 79 totheir engaged position. The apparatus is new in a condition to make asecond attempt to establish flame at the pilot burner 12 and the mainburner it if there is still a need for such operation as evident by thethermostat 24.

Operation on false indication of flame It will now be assumed that thethermostat 24 has been satisfied and has caused the connection fromterminal 25 to terminal 26 to be broken. Relay 65 is thereby deenergizedand causes the valve 111, the valve 13 and the ignition'transformer 15to be deenergized.

,It will also be assumed that the relay 47 does not respond to theabsence of flame at the pilot burner 12 and the main burner it) causedby the deenergization of the valves 13 and 11 respectively. Theapparatus has now returned to the position shown in the single figurewith the exception that the relay 47 remains energized to falselyindicate the presence of flame. Under these conditions an energizingcircuit can be traced for th heater 77 of the safety cut-out device fromthe tap 232 of the secondary 81 through conductor 144-, heater 77,conductor M3, switch blade 49 and contact 54 of relay 47, conductor 180,contact 70 and movable switch blade 67 of relay 65, and conductor 151 tothe bottom terminal of the secondary 81. After a predetermined length oftime the safety cut-out device 75 will be actuated to causedisengagement of the contacts 78 and 79. The thermostat 24 can thereforeno longer cause energization of the relay 65 since the energizingcircuit for relay 65 is open at safety cut-out contacts 78 and '79.

So long as this faulty condition exists and relay 47 falsely indicates apresence of flame, depressing actuator 80 of the safety cut-out device75 does not reset the contacts 78 and 79 to their engaged positionssince the heater 77 will remain energized by means of the above tracedcircuit, which circuit will remain energized as long as relay 47 falselyindicates the presence of flame. In or der to again establish flame atthe main burner 19 it is necessary for the fault in the system to becorrected so that the flame relay 47 again returns to its no flameposition as shown in the single figure. The bimetal 77 then cools andcut-out device is in a condition to be reset by means of reset actuator8t It can therefore be seen that I have shown an improved burner controlapparatus capable of sensing the fluctuating characteristics of theflame and having a simple and inexpensive filter network connected torender the burner control apparatus frequency responsive to thecharacteristic fluctuations of the flame.

I claim as my invention:

1. A flame detector for use with a fuel burner which supports a flamefluctuating in intensity in a characteristic frequency range when fuelis supplied thereto, comprising: a. photoelectric means sensitive to thepresence of a flame; an amplifier having an input stage and an outputstage adapted to control a burner control means; means connecting saidphotoelectric cell to the input stage of said amplifier; an intermediatestage within said amplifier means having an input and an output circuitwith said circuits connected to a. reference potential level; and a nullnetwork connected to form a feedback circuit from said input circuit tosaid output circuit of said intermediate stage; said network including afirst resistor directly connected in an alternating current sense at oneend to said input circuit and at the other end to said output circuit,two series connected capacitors connected in parallel with said firstresistor, and a second resistor connected from the common connection ofsaid capaciters to said reference potential level; said network therebycausing said intermediate stage to reject all signal voltages not withina predetermined band of frequencies substantially identical to thecharacteristic frequency range of the flame by producing degeneration ofall signal voltages not within said predetermined band of frequencies.

2. A flame detector for detecting the characteristic fluctuations inintensity of the infrared radiations emitted by a flame and for use witha burner which supports a flame when fuel is supplied thereto,comprising: a photoconductive cell sensitive to infrared wave energy,said cell varying in conductivity in accordance with the fluctuations inthe intensity of infrarer radiation impinging upon said cell when saidcell is exposed to a flame; a direct current source of power; a loadimpedance; means connecting said source of power, said load impedanceand said photoconductive cell in series whereby a fluctuating signal isdeveloped across said load impedance when said cell is exposed to aflame; amplifier means having an input stage, an intermediate stagehaving an input and an output circuit connected to a reference potentiallevel, and an output stage adapted to control the energization of aburner control means; means connecting said photoconductive cell to theinput stage of said amplifier means, and a null feedback networkassociated with said intermediate stage to thereby cause saidintermediate stage to be degenerative to all signals not Within therange of the characteristic fluctuations of a flame, said null networkincluding a first resistor directly connected in an alternating currentsense from the input to the output circuit of said intermediate stage,two series connected capacitors connected in parallel with said firstresistor, and a second resistor connected from the common connection ofsaid two capacitors to said reference potential level.

3. A flame detector for detecting the characteristic frequency range offluctuation in intensity of the infrared wave energy emitted by a flameand for use with a fuel burner, comprising; a photoconductive cellsensitive to infrared wave energy, said cell varying in conductivity inaccordance with the characteristic fluctuations in intensity of theinfrared wave energy when said cell is exposed to a flame, a source ofdirect current power, a load resistor; means connecting said source ofpower, said load resistor and said photoconductive cell in a seriescircuit whereby a fluctuating signal voltage is developed across saidload resistor when said cell is exposed to a flame; amplifier meanscomprising an electron discharge device having an anode, cathode andcontrol electrode; means connecting said load resistor through acoupling capacitor to said control electrode; means connecting saidcathode to a reference potential; a bridge T null network associatedwith said electron discharge device to form a frequency selectivedegenerative feed-back circuit which is effective to cause saidamplifier means to substantially reject all fluctuating control voltagesapplied to said control electrode which do not lie within thecharacteristic frequency range of fluctuation of a flame, a firstcapacitive leg of said T network connected to said control electrodethrough said coupling capacitor, a second capacitive leg connecteddirectly to said anode, a third resistive leg connected to saidreference potential and a bridge resistor connected directly to saidanode and through said coupling capacitor to said control elec-v trodeand means connected to said anode to be energized when said fluctuatingsignal voltage applied to said control electrode lies within saidcharacteristic frequency range and adapted to control burner controlmeans.

4. A flame detector for use with a fuel burner which supports a flameemitting infrared wave energy fluctuating in intensity in acharacteristic frequency range comprising, a photoelectric cellsensitive to infrared wave energy, an amplifier having an input stage,an intermediate stage and an output stage; means energized by saidoutput stage when a control signal is applied to said output stage andadapted to control burner control means; means connecting saidphotoelectric cell to said input stage to thereby apply a fluctuatingcontrol signal of a characteristic frequency range to said input stagewhen said photoelectric cell is exposed to a flame; a direct currentsource of power; an electron discharge device in said intermediate stageand having a cathode, anode and control electrode; means connecting saidcathode to a reference potential level; means connecting said anodethrough a load impedance to said direct current source of power; meansconnecting the connection of said load impedance and said anode to saidoutput stage; means connecting said control electrode to said inputstage through a coupling capacitor, and a bridge T null networkconnected to form a degenerative feedback circuit for said intermediatestage, said bridge T network having a first capacitive leg connected tosaid control electrode through said coupling capacitor, a secondcapacitive leg connected to the connection of said load impedance andsaid anode, a third resistive leg connected to said reference potentiallevel and resistance means connected from the connection of said loadimpedance and said anode to said control electrode through said couplingcapacitor, said intermediate stage thereby being degenerative to allcontrol signals not within said characteristic frequency range toeffectively prevent all such control signals from affecting said outputstage.

5. A flame detector for use with a fuel burner to detect a flame at theburner by sensing a characteristic fluctuation in the flame, comprising:flame sensing means for sensing the characteristic fluctuation in aflame; an electronic amplifier having a plurality of stages; meansconnecting said flame sensing means to the input stage of said amplifierto apply a control signal of said characteristic fluctuation to saidamplifier when said flame sensing means is exposed to a flame; a furtherstage for said amplifier having an input circuit connected to areference potential level and an output circuit including a loadimpedance connected to the output of said further stage and to saidreference potential level, means connecting said input stage to theinput circuit of said further stage through a coupling capacitor, saidfurther stage having a bridge 1 null network associated therewith toform a frequency selective degenerative feedback circuit for saidfurther stage to cause said further stage to be degenerative to allcontrol signals not within the range of characteristic fluctuation of aflame, said bridge T network having a first capacitive leg connectedthrough said coupling capacitor to the input circuit of said furtherstage, a second capacitive leg connected to the connection of said loadimpedance to the output of said further stage, a third resistive legconnected to said reference potential, and bridge resistive means havingone end connected to the connection of said load impedance to the outputof said further stage and the other end connected through said couplingcapacitor to the input circuit of said further stage; and means adaptedto connect the output circuit of said further stage to control burnercontrol means, said last named means being effective only when thecontrol signal applied to said input stage is within the range ofcharacteristic fluctuation of a flame.

6. A flame detector for use with a fuel burner which supports a flamewhen fuel is supplied thereto, comprising, flame sensing means sensitiveto fluctuation in the intensity of electromagnetic Wave energy emittedby a flame to supply a fluctuating control signal indicative of thepresence of flame, amplifier means having an input and an output andhaving a reference potential, bridge T filter means associated with saidamplifier means to render said amplifier means frequency selective toonly that range of frequencies including said fluctuation in wave energyemitted by a flame, said filter means having a first capacitive legdirectly connected to the output of said amplifier means and a secondcapacitive leg directly connected in an alternating current sense to theinput of said amplifier means, a third resistive leg connected to saidreference potential, and resistor bridging said first and second legs,means connecting said photoelectric flame sensing means in controllingrelation to the input to said amplifier, and means rendered operativeupon a control signal within said range of frequencies being supplied tothe input of said amplifier means and adapted to control burner controlmeans.

References Cited in the file of this patent UNITED STATES PATENTS2,245,365 Riddle June 10, 1941 2,304,641 Jones Dec. 8, 1942 2,372,419Ford et al Mar. 27, 1945 2,383,984 Oberweiser Sept. 4, 1945 2,499,921Hurley Mar. 7, 1950 2,499,996 Kelsey Mar. 7, 1950 2,632,102 JellinekMar. 17, 1953 2,692,962 Thomson Oct. 26, 1954

